The present invention relates to a PSK (phase shift keying) demodulator, and more particularly to a PSK demodulator having a simplified carrier wave reproducing circuit.
A conventional, demodulator of this kind has a carrier wave reproducing circuit for reproducing a carrier wave from an N-phase (N=2.sup.n, n being a positive integer) PSK-modulated carrier wave, and an N-phase PSK-demodulating circuit for providing demodulated signals by detecting the PSK-modulated carrier wave with this reproduced carrier wave. Such a PSK demodulating circuit generally includes as its components n phase detectors and n level decision circuits together with (n-1) phase shifters. A first example of conventional carrier wave reproducing circuits (see Reference 1, William C. Lindsey et al., Telecommunication Systems Engineering, 1973, Prentice-Hall, pp. 55-81) comprises an N-fold frequency multiplier for frequency-multiplying by N an N-phase PSK carrier wave to generate an unmodulated carrier wave having a frequency N times as high as the carrier wave frequency, a phase synchronizing circuit or a PLL (phase locked loop) circuit for providing an unmodulated wave phase-synchronized with this unmodulated carrier wave, and a 1/N frequency divider for dividing the phase-synchronized wave by N to generate the carrier wave. A similar circuit is disclosed in the U.S. Pat. No. 3,835,404. Such wave reproducing circuit requires circuits operable in a frequency band of Nxfc higher than the carrier wave frequency band fc of the N-phase PSK carrier wave. Therefore the circuits have to meet increasingly severe requirements with an increase in number of phases, resulting in a greater cost and sometimes in the unavailability of elements operable in the required high frequency band. These disadvantages may be obviated by converting the N-phase PSK carrier wave into a lower frequency band by means of a suitable local oscillator and then supplying the converted carrier wave to the aforementioned first carrier wave reproducing circuit to enable this circuit to operate in the carrier wave band. This circuit composition, however, requires a separate local oscillator, in particular one with a small frequency drift, i.e., a highly stable local oscillator, to minimize the stationary phase error in the PLL circuit (including a voltage-controlled oscillator, a phase comparator and a low pass filter), inviting an increase in cost.
A second example of conventional carrier wave reproducing circuits (see Reference 2, U.S. Pat. No. 4,110,706) consists of an N-phase PSK modulating circuit involving n phase detectors which inversely modulates an N-phase PSK carrier wave with a demodulating signal and (n-1) phase shifters, a phase-synchronizing circuit (or a PLL circuit) for generating a carrier wave phase-synchronized with the output of this modulating circuit, and a delay circuit for adjusting the delay time on each path. This carrier wave reproducing circuit, wherein the delay achieved by the delay circuit varies with the ambient temperature and other factors as stated in the reference, is incapable of proper phase synchronization. Therefore, the PSK demodulating circuit is unable to demodulate correct signals from the PSK-modulated carrier wave. There is another disadvantage that the structure of the PSK-modulating circuit enlarges with an increase in number of phases. The carrier wave reproducing circuit of the Costas loop has a similar shortcoming.